prohibit to run freeMemoryIfNeed when it is slave

Situation: when Slave's maxmemory and maxmemory-policy are different from Master's
some datum are in Master but they aren't in Slave.

How:
for consistency between master and slave. it is better to turn off slave's freememoryIfNeed.

Why::
1] at the first time, I considered to copy master's maxmemory and maxmemory-policy setting
when they sync for the first time. but it can cause some troubles. because Master and Slave uses memory differently. 

2] even though, they uses same maxmemory-policy, slave can perform "LRU or Expire" itself.

In conclusion, Skipping freememoryIfNeed in slave is better.